1. Field of the Invention
The invention relates generally to the field of switching power converters, and more particularly, to control methods that enable power converters to achieve reduced output voltage ripple and high light-load efficiency while maintaining quiet operation, free from audible noise.
2. Description of Related Art
A switching power converter control system, typical of the prior art, can be characterized as constant on-time control. FIG. 1 is a simplified schematic of a boost converter employing constant on-time control. Vin is the input voltage 102, and Vo is the output voltage 112. The input inductor 104 has an equivalent series resistance (ESR) of RL, represented by resistor 106. Under the constant on-time control method, the MOSFET 108 is turned on for a duration Ton, and then turned off for a duration Toff that is determined by a feedback voltage that is developed by the resistive divider formed by resistor 116 and resistor 118. This feedback voltage 126 is routed to comparator 122, where it is compared with reference voltage 120. When the feedback voltage 126 is less than the reference voltage 120, flip flop 124 turns MOSFET 108 on for a time duration Ton. When the feedback voltage 126 rises above reference voltage 120, the MOSFET 108 is switched off. For a given load and given values of Vin and Vo, the circuit operates in a steady state mode with frequency fs and period Ts, where fs=1/Ts and Ton=D*Ts, where D is a duty cycle ratio less than one. For an input inductor 104 having a value of L and a change in inductor current at MOSFET 108 turn on of diL, the governing equation of the boost converter can be written as:Vin*Ton=L*diL=(Vo−Vin)*Toff.
In steady state operation, when the boost converter operates in continuous conduction mode (CCM), Ton+Toff=T. However, in discontinuous conduction mode (DCM), Ton+Toff<Ts. From the equation above, it can be seen that for a given Vin and L, if Ton is fixed, diL will take on a fixed value. If the load current is reduced and the circuit enters DCM operation, the Toff time will increase in order to maintain the regulation of the output voltage Vo 112. When the load current is further reduced, the operating frequency of the circuit will decrease, reducing switching losses and improving efficiency under light load. When the load current approaches zero, the boost circuit under constant on-time control may maintain switching at a low frequency such as 5 kHz, or it may enter a hiccup mode, also known as a burst mode. When a regulator enters burst mode, it may produce audible noise and exhibit increased output voltage ripple.
As an alternative to constant on-time control, some systems of the prior art use a constant off-time control mode in conjunction with burst mode operation. In such systems, a fixed delay circuit maintains a constant Toff time, while the on time changes as a function of the input and output voltages and the load. In either the constant-on-time or constant-off-time control systems, the typical inductor-current and output-voltage waveforms are pulsed, which improves efficiency, but has the drawback of increased output ripple and audible output noise. FIG. 2 illustrates the typical operation of a system using constant-off-time control that has entered burst mode due to a large drop in output load current. Time is plotted along horizontal axis 210. In this exemplary plot, the timescale is ten microseconds per division. The lower waveform 202 illustrates the output voltage, with the vertical axis 212 corresponding to one hundred millivolts per division. The upper trace 204 is the inductor current, plotted at 0.2 Amperes per division. Burst mode operation can be seen in both the current and voltage traces, for example at 206. This voltage ripple at the output and audible noise that may be produced may both be disadvantageous in many applications. Accordingly, it would be useful to produce a system that would remain free from such noise even under very low output-load current conditions.